chore(docs): prevent theme flash by respecting system preference on initial load (#3840)

* fix:system pref fix

* fix: fixes demo page light mode error

* Update docs/demo/demo.js

Co-authored-by: Steven <steven@ceriously.com>

* Update docs/js/index.js

Co-authored-by: Steven <steven@ceriously.com>

* Update docs/demo/demo.js

Co-authored-by: Steven <steven@ceriously.com>

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Co-authored-by: Steven <steven@ceriously.com>